The present invention relates generally to prosthetic joints and, more particularly, to modular orthopaedic knee implant systems.
The knee joint basically consists of the bone interface of the distal end of the femur and the proximal end of the tibia. Appearing to cover or at least partially protect this interface is the patella which is a sesamoid bone within the tendon of the long muscle (quadriceps) on the front of the thigh. This tendon inserts into the tibial tuberosity and the posterior surface of the patella is smooth and glides over the femur.
The femur is configured with two knob-like processes (the medial condyle and the lateral condyle) which are substantially smooth and which articulate with the medial plateau and the lateral plateau of the tibia, respectively. The plateaus of the tibia are substantially smooth and slightly cupped thereby providing a slight receptacle for receipt of the femoral condyles.
When the knee joint is damaged whether as a result of accident or illness, a prosthetic replacement of the damaged joint may be necessary to relieve pain and to restore normal use to the joint. Typically, the entire knee joint is replaced by means of a surgical procedure which involves removal of the surfaces of the corresponding damaged bones and replacement of these surfaces with prosthetic implants. This replacement of a native joint with a prosthetic joint is referred to as primary total-joint arthroplasty.
On occasion, the primary joint prosthesis fails. Failure can result from many causes, including wear, aseptic loosening, osteolysis, ligamentous instability, arthrofibrosis and patellofemoral complications. On such failure, a revision procedure may be necessary. In a revision, the primary joint prosthesis is removed and replaced with components of a revision joint system.
Implant systems for both primary and revision applications are available from a variety of manufacturers, including DePuy Orthopaedics, Inc. of Warsaw, Ind. DePuy and others offer several different systems for both primary and revision applications. For example, for the knee joint, DePuy Orthopaedics offers the P.F.C. SIGMA® Knee System, the LCS® Total Knee System and the S-ROM Modular Total Knee System. Each of these orthopaedic knee systems includes several components, some appropriate for use in primary knee arthroplasty and some appropriate for use in revision surgery.
An example of the existing DePuy P.F.C.SIGMA Knee System is illustrated in FIGS. 1-4. As shown in FIG. 1, a typical DePuy P.F.C. SIGMA modular orthopaedic knee implant system 8 includes a modular femoral implant 10 and a modular tibial implant 11. The modular femoral implant 10 includes a distal femoral component 12, an elongate femoral stem member 14, a bolt 16 and a femoral stem collar 18. The modular tibial implant 11 includes a tibial tray 13, a tibial insert 15 and an elongate tibial stem extension 17.
The modular femoral component 12 includes two spaced condylar portions 20, 22 with articulating surfaces 24, 26 to engage articulating surfaces 23, 25 of the tibial insert 15. As shown in FIGS. 2-3, on the proximal side of the femoral component 12, the two condylar portions 20, 22 are connected by an intercondylar box or boss 28. The intercondylar box or boss 28 has a pair of substantially vertical side walls 30, 32 that are connected by a top or superior seating or mounting platform 34. The boss mounting platform 34 is generally planar, and has an opening 36 (see FIG. 4) that extends through the boss to define an open channel. The bolt 16 extends through the channel from the distal side of the femoral component and through the opening 36 to be connected to a female threaded opening in the femoral stem 14. For clarity, the bolt's external threads are not shown on the drawings, but it should be understood that the proximal end of the bolt 16 is threaded.
The femoral stem collar 18 has a male portion 37 (FIG. 4) for connection to a distal female threaded end of the stem member 14. A retaining ring is used to hold the components 14, 37 together. Thus, the femoral stem member 14, stem collar 18 and femoral component 12 can be assembled to secure the stem member 14 to the femoral component 12. With this design, a variety of styles and sizes of stem members and femoral components can be assembled to best suit the patient's anatomy and joint conditions. For example, an implant kit could include a set of different sizes of stem members with outer surfaces adapted for cemented implantation as well as with fluted outer surfaces.
As shown in FIG. 3, when assembled, the femoral stem member 14 is typically angled in a medial direction. The angle is labeled in FIG. 3 as α. The angle α is between the axis 40 of the femoral stem member 14 and a line 41 perpendicular to the plane 42 of the seating or mounting platform 34 of the intercondylar boss or box 28. The line 41 is also generally co-linear with the central longitudinal axis of the tibial stem extension 17. The angle α corresponds with the valgus angle when the implant assembly is implanted; the valgus angle is defined as the angle between the center line of the femur and the vertical axis connecting the distal femur and the center of the femoral head; the center line of the femur will correspond with the axis 40 of the stem member 14, and the vertical axis connecting the distal femur and the center of the femoral head will correspond with the line 41.
In the illustrated prior art modular assembly, the angle α is set by the structure of the femoral stem collar 18. The femoral stem collar has a superior side or surface 33 lying in a plane and an inferior side or surface 35 lying in a plane that is not parallel to the plane of the superior side or surface 33. The inferior surface 35 of the collar 18 is angled, defining an obtuse angle (90°+α) with the axis 40 of the stem.
For the femoral side, a typical existing implant kit for DePuy's P.F.C. SIGMA Knee System includes: four sizes of femoral stem members 14 for use in cemented applications, with diameters of 13 mm and 15 mm and lengths of 90 mm and 130 mm; sixteen sizes of fluted femoral stem members 14 for non-cemented use, with diameters of 10 mm, 12 mm, 14, mm, 16 mm, 18 mm, 20 mm, 22 mm and 24 mm and lengths of 125 mm and 175 mm in each diameter; two bolts, a standard bolt and one providing a 2 mm anterior-posterior offset; and two stem collars 18 providing angles of 5° and 7° for the angle α, although angles α may be in a typical range of 5-9°. A variety of sizes of distal femoral components 12 are also included in the typical commercial kit. With this variety of modular components, the surgeon can customize the femoral side of the prosthesis to best fit the needs of the individual patient.
Although not shown in FIGS. 1-3, the illustrated prior art stem collar 18 has a central bore to receive part of the bolt 16. The central bore has a central longitudinal axis defining an obtuse angle with at least one of the planes of the superior and inferior sides 33, 35 of the stem collar 18.
To ensure that the angle α remains in the illustrated orientation, the intercondylar box or boss 28 typically has a pair of anti-rotation tabs 46, 48 (see FIGS. 2 and 4) that mate with opposing flats 50, 52 on the femoral stem collars 18.
As commercially supplied, the stem members 14 and stem collars 18 are supplied as a unit, connected together prior to being supplied to the surgeon.
As shown in FIG. 1, on the tibial side, the tibial tray 13 of the modular tibial implant 11 comprises a tibial platform 53, with an integral stem 54 and integral keels 56 extending between the distal side of the tibial platform and the stem 54. The tibial platform 53 and stem 54 may have aligned bores to receive a distal extension of the tibial insert 15. In FIG. 1, the illustrated tibial platform and insert are configured for a fixed bearing, although it should be understood that these components could be configured so that the insert bearing 15 can rotate.
The distal end 58 of the tibial stem 54 has an interior surface with a threaded female opening (not shown). The threaded female opening mates with a plug (not shown) that can be used where a stem extension is not necessary, and can also be used with the illustrated tibial stem extension 17. The tibial stem extension 17 has a male threaded end 60 sized and shaped to mate with the threaded female opening at the distal end 58 of the stem 54.
For the tibial side, a typical existing implant kit for DePuy's P.F.C. SIGMA Knee System includes: four sizes of tibial stem extensions 17 for use in cemented applications, with diameters of 13 mm and 15 mm and lengths of 30 mm, 60 mm, 90 mm, 120 mm and 150 mm; twenty-four sizes of fluted tibial stem extensions 17 for non-cemented use, with diameters of 10 mm, 12 mm, 14, mm, 16 mm, 18 mm, 20 mm, 22 mm and 24 mm and lengths of 75 mm, 115 mm and 150 mm in each diameter. A variety of sizes of modular tibial trays 13 and inserts 15 are also included in the typical commercial kit. With this variety of modular components, the surgeon can customize the tibial side of the prosthesis to best fit the needs of the individual patient.
Variations on the knee implant system illustrated in FIGS. 1-4 have been disclosed. For example, posterior stabilized mobile bearing knees are disclosed in U.S. Pat. Nos. 6,727,723 B2 and 6,443,991. Adapters for such knee implant systems are disclosed in the following U.S. Pat. Nos. 6,171,342B1; 5,824,097; 5,782,921; and 5,182,921. U.S. Pat. Nos. 5,683,472 and 6,126,693 also disclose features related to knee implant systems. The disclosure of each of these patents is incorporated by reference herein in its entirety.
Although these prior knee implant systems have provided surgeons with great flexibility in meeting patient needs, and the modularity of these systems provides the opportunity to reduce the number of components needed in a surgical kit, these systems still require a substantial number of components.